A carbonylation process of a methanol method (an acetic acid process of a methanol method) is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The reaction mixture is evaporated in an evaporator, and the vapor phase is purified in a lower boiling point component removal column and subsequently in a dehydration column so that product acetic acid is prepared. Alternatively, product acetic acid is prepared via a higher boiling point component removal column subsequent to the dehydration column, and further, a product column.
In such an acetic acid production process, acetaldehyde is produced as a by-product in a reaction system, and the aldol condensation of acetaldehyde produces crotonaldehyde. Crotonaldehyde causes deterioration in a potassium permanganate test value (permanganate time) of product acetic acid. Furthermore, crotonaldehyde reacts with acetaldehyde to produce 2-ethyl crotonaldehyde. Although 2-ethyl crotonaldehyde also causes deterioration in the potassium permanganate test value of product acetic acid, the degree of potassium permanganate test deterioration per mass of 2-ethyl crotonaldehyde is much smaller than that of crotonaldehyde. Conventionally, the following two general methods have been industrially adopted in order to decrease crotonaldehyde or 2-ethyl crotonaldehyde: (i) a method for separating and removing acetaldehyde produced as a by-product in a reaction system from methyl iodide in a purification step, and decreasing acetaldehyde in methyl iodide to be recycled to the reaction system to suppress the production of crotonaldehyde in the reaction system; and (ii) a method for directly subjecting crotonaldehyde contained in crude acetic acid obtained during a purification step to oxidative decomposition using ozone (Patent Literatures 1 and 2). However, both separation and removal equipment of acetaldehyde and ozonation equipment are expensive. Conventionally, an increase in the potassium permanganate test value of product acetic acid has thoroughly depended on these methods, which has led to an increase in the equipment cost.